If you want more top speed you want to increase your gear box sprocket size. This will lower the amount of torque you have but increase the top speed. If you want more power and less top speed then go with a smaller sprocket for the gear box. For example if you have a 7 tooth gear and your top speed is 30mph, you change your gear to a 6 tooth and your top speed will be about 27mph but you will gain power!